The Electronic Age - The Science of Navigation - page 209

via these three antennas depends on the orientation angle, a , of the an-
tenna wheel relative to the transmitter direction. The dependence of re-
ceiver power on wheel angle a is shown in figure 8.4b. There is a peak
in the receiver output signal at zero angle when the wheel is oriented
with antennas 1 and 2 equidistant from the transmitter. Thus, my simple
receiver—let us call it a Mercedes receiver —acts as an RDF: it finds the
direction of an NDB transmitter and so provides a navigator with an LoP.
But also note the width of this Mercedes receiver beam shown in fig-
ure 8.4b. Such widths are conventionally measured in terms of the angular
extent of the beam at 50% of peak power; in this case the beam is about 50\
wide. In principle, a navigator could manually adjust the receiver wheel of
figure 8.4a until it pointed in the exact direction of peak power, but in
practice, there would always be an error: it is not possible to estimate the
peak angle exactly. Radio engineers found that it was easier to precisely
detect nulls (angles for which they received zero power) than peaks. As a
result, real antennas were designed so that, unlike the example of fig-
ure 8.4, the receiver emitted zero signal only when it was aligned with the
transmitter.
Another problem inherent in the design of my Mercedes RDF receiver
is seen if we extend the beam pattern of figure 8.4b—the graph of average
power received versus orientation angle—to larger angles. The same peak
is displayed at angles of 120\ and 240\, so a navigator who used this type of
receiver would have to derive some method for deciding which of three
directions that produce an output peak corresponded to the true direction
of the transmitter.
A very common and simple receiver antenna used in real systems (un-
like my hypothetical Mercedes receiver) was the loop antenna . It produced
a null output signal if the plane of the loop was perpendicular to the
direction of the transmitter. This design meant that a navigator had to
choose one of two nulls separated by 180\ (instead of three peaks sepa-
rated by 120\, as for the Mercedes design). Supplementary circuitry sorted
out this ambiguity. 9 Because loop antennas could be small, these anten-
9. Despite the relative ease of sorting out the 180\ ambiguity of transmitter direction—
which was due to the symmetry of the loop antennas (they looked the same from the back as
from the front)—there were still serious navigational accidents that resulted from misiden-
tifying the true direction of the transmitter. In 1923 a squadron of 23 U.S. destroyers ran
aground on the coast of southern California because of this elementary error, and 6 ships
were lost. See Cutter (2004) for this detail and for a summary of early RDF developments in
marine navigation. RDF is still practiced as a hobby by radio enthusiasts around the world.